Abstract: Apparatus for measuring the Young's modulus or the internal damping coefficient of skin, for example human skin, in vivo, comprises a rectangular frame of non-metallic material to be adhered by means of double-sided adhesive tape to the surface of skin to be tested, and a movable assembly also adherable to the skin surface by means of tape.The frame carries a vibration exciter in the form of a coil co-operating with a magnetic bead on the movable means, and also an opto-electronic movement sensor comprising an optical transmitter and receiver mounted next to one another on the frame and an optical reflector on the movable means for reflecting back to the light receiver the beam emitted by the light transmitter.